This invention relates to the use of a catalyst slurry for hydrotreating heavy fossil fuel feedstocks such as vacuum gas oils or heavy gas oils. High catalyst activity is maintained by circulating the catalyst between a hydrotreating zone and a hydrogen stripping reactivation zone.
The petroleum industry employs hydrotreating to process heavy vacuum gas oils, particularly coker gas oils, in order to improve their quality as fluid catalytic cracker (FCC) feeds. Hydrotreating accomplishes the saturation of multi-ring aromatic compounds to one-ring aromatics or completely saturated naphthenes. This is necessary to assure low coke and high gasoline yields in the cat cracker. Multi-ring aromatics cannot be cracked effectively to mogas and heating oil products, whereas partially hydrogenated aromatics or naphthenes can be cracked to premium products. Hydrotreating is further capable of removing sulfur and nitrogen which is detrimental to the cracking process.
Hydrotreating employs catalysts that tend to become poisoned by organic nitrogen compounds in the feed. Such compounds become adsorbed onto the catalyst and tie up needed hydrogenation sites due to the slow kinetics or turnover for hydrodenitrogenation. Higher temperatures may be utilized to overcome this problem. However, at high temperatures thermodynamic equilibrium tends to favor the preservation of undesirable multi-ring aromatic compounds.
It is an object of the present invention to circumvent both the kinetic and equilibrium limits encountered in conventional hydrotreating processes which employ fixed bed catalysts. It is a further object of the present invention to provide an improved hydrotreating process employing a catalyst slurry. It is a still further object of the present invention to accomplish reactivation of the catalyst employed in the present process by hydrogen stripping the catalyst in an essentially continuous cyclic process.
In comparison to the present process, hydrogen stripping with a conventional fixed bed reactor has been found to provide only a temporary gain in catalyst activity, which gain is quickly lost in a few days. Therefore, frequent and expensive shut downs would be required for hydrogen stripping to be effective in a fixed bed hydrotreating process.
Hydrotreating processes utilizing a slurry of dispersed catalysts in admixture with a hydrocarbon oil are generally known. For example, Pat. No. 4,557,821 to Lopez et al discloses hydrotreating a heavy oil employing a circulating slurry catalyst. Other patents disclosing slurry hydrotreating include U.S. Pat. Nos. 3,297,563; 2,912,375; and 2,700,015.
Various problems in operating the slurry processes disclosed in the prior art have apparently hindered commercialization. For example, according to the process disclosed in Pat. Nos. 4,557,821; 2,912,375 and 2,700,015, it is necessary to reactivate the catalyst by air oxidation. However, air oxidation is expensive since depressurization of the catalyst environment between the hydrotreating reactor and the reactivator, requiring expensive lock hoppers, is necessary before combusting off the contaminants on the catalyst. Furthermore, expensive equipment is necessary to avoid air contamination and possible explosions.